This invention generally relates to variable gain circuitry. More specifically, this invention relates to a variable gain element for adjusting an amplitude of an input signal.
Cellular telephone usage has continued to increase in popularity. Cellular telephone manufacturers constantly strive to improve the performance of their products to gain market share. Specifically, manufacturers try to minimize the energy necessary to power the cellular telephone; this reduces battery drain and thus increases the available talk time of a cellular telephone on a single battery charge. Talk time is a critical performance specification that consumers use to compare various cellular telephones on the market.
Manufacturers are also constantly striving to improve the appearance of their products. Thus, manufacturers are always looking for ways to reduce the size of the cellular phones because consumers desire cellular telephones that are small and easy to carry.
Yet another design goal is to minimize the cost of the cellular telephone. A manufacturer can gain a significant competitive advantage if it can design a functioning cellular telephone at a low cost.
To meet the needs of minimizing battery drain, reducing the size of cellular telephones, and minimizing the manufacturing cost, more and more of the electrical circuit functions are accomplished through the use of integrated circuit technology.
Much of the functionality of the cellular telephone transceiver are embedded in integrated circuits. One key circuit block of both the receiver and the transmitter is a variable gain element. This can be in the form of a variable gain amplifier and/or variable gain attenuator.
For example, in a conventional cellular telephone transmitter, at least one variable gain element is needed to vary the transmitted output power in accordance with the cellular telephone standard. A cellular telephone that is close to a base station does not have to transmit as much power as a cellular telephone further away from the base station.
FIG. 1 shows a prior art variable gain element 10 suitable for integrated circuit technology. In any particular transmitter or receiver, the variable gain element 10 can be used at a radio frequency (RF), at an intermediate frequency (IF), or both.
At the core if variable gain element 10 is a pair of emitter coupled transistors, Q1 and Q2. A differential signal input is applied to input ports 18 and 20. A DC current source 16 couples the emitters of Q1 and Q2 to ground.
The collectors of Q1 and Q2 are each connected to emitter coupled differential pairs. For example, in the collector of Q1 there is an emitter coupled pair Q3 and Q4, and in the collector of Q2 there is an emitter coupled pair Q5 and Q6. The base terminals of Q4 and Q5 are connected together at port 32, where a DC reference voltage is applied. The base terminals of Q3 and Q6 are connected together at control port 30, where a DC control voltage is applied. The collector terminals of Q3, Q4, Q5, and Q6 are each coupled to supply voltage 36 through separate resistors. The attenuated or amplified signal is coupled from the collector of Q3 at output port 34. If a differential output is desired, the complementary output signal can be coupled from the collector of Q6.
In operation, the differential input signal is applied to input ports 18 and 20. The gain of the Q1/Q2 differential pair is related to gm*R as is known in the art. However, the gm here is manipulated by steering current away from load resistor 40 to decrease the gain (e.g. attenuate) or to load resistor 40 to increase the gain. This current steering is accomplished by altering the DC control voltage applied to control port 30. For example, as the DC control voltage increases above the DC reference voltage at port 32, the gain increases and the output signal increases in magnitude. Conversely, as the DC control voltage decreases below the DC reference voltage at port 32, the gain decreases and the output signal appearing at output port 34 decreases in magnitude.
The variable gain element 10 has several drawbacks. First, the noise performance varies as a function of gain. For example, at maximum gain (Vcntl greater than Vbias), transistors Q3 and Q6 are fully on, while Q4 and Q5 are essentially turned off. As the attenuation gain is decreased by about 6 dB (e.g. Vcntl reduced), transistors Q3, Q4, Q5, and Q6 are all conducting and thus contribute to the overall noise performance of the variable gain element 10. As the gain if further decreased to a minimum gain (e.g. Vcntl lowered below Vbias), only transistors Q4 and Q5 are conducting, and the noise power drops. Thus, there is a peaking in the noise power produced from the variable gain element 10.
A second drawback relates to the intermodulation performance of the variable gain element. The intermodulation components of the variable gain element 10 peak at about a 6 dB cutback in the gain, and this degradation in the intermodulation performance degrades the overall performance of the transceiver. Thus, there is a need for a variable gain element suitable for integrated circuit implementation that has improved noise and intermodulation performance.